Tissue factor (thromboplastin) will be purified by fractionation of detergent extracts of delipidated bovine brain and human placenta. Tissue factor will be recovered from SDS polyacrylamide gels and procoagulant activity will be restored by renaturation from detergent. The tissue factor band will be used as an immunogen. Antibodies to this material will be used for histochemical localization of tissue factor in tissues and in cultured cells. A radioimmunoassay will be used to follow the development of tissue factor in cultured cells under a variety of conditions. The purified protein will be characterized chemically and stimulation of activity by phospholipids with different physical properties will be studied. The stoichiometry of purified tissue factor in the conversion of factor X by factor VII will be studied kinetically. Crosslinking experiments will demonstrate whether the basic mechanism for tissue factor activity is destabilization of the factor VII-factor X enzyme-substrate complex. The smallest fragment of tissue factor with catalytic activity will be generated by proteolytic digestion of the purified protein; chemical modification will identify amino acid residues required for full activity.